1. Field of the Invention
The present invention relates to a focus detection unit and to a microscope comprising the focus detection unit. This focus detection unit employs a distinguishing image sandwiching method.
2. Description of the Related Art
Means for achieving accurate and reliable detection of optical instrument focus has long been studied. In an example case where a measurement microscope is manually adjusted to focus on the surface of an object and that microscope has a low magnification with generally low detection accuracy due to deep depth of focus, either an ocular or an objective having a large numerical aperture (NA) for high magnification are generally used to increase the magnification of the microscope or to reduce the depth of focus.
In order to achieve more reliable focusing, a method for image split has recently been employed. This method is performed using a split prism such as is shown in FIG. 9.
The optical system of a reflected illumination microscope employing this method incorporates a target 72 and a split prism 70, which together constitute an accurate focusing mechanism. This method is based on the optical principle that the upper and lower split images of a target are sharply shown without displacement between each other in the middle of the sharp image of a workpiece when focus is properly adjusted and that they are shown horizontally displaced when focusing is not properly adjusted. That is, for focus adjustment in this method, whether a vertical line in the image of a target appears straight (position 200) or horizontally displaced (position 202) is observed instead of examining a blur condition of the image surface of a workpiece. With this method, more accurate focusing than conventional methods is achieved as this method has nothing to do with the depth of focus of the objective 76 and, moreover, utilizes matching error recognition which human eyes are more sensitive than to two-point recognition.
However, split prisms used for this method have complicated structures, and accordingly have a large size in order to cover the entire workpiece. This not only creates a sliding vertical line in the center of a target image, but also makes it difficult to correct an out-of-focused state (the position 202).
In order to solve the above problems, an image sandwiching method has also been proposed. In this method, a single line sandwiched by two other lines is utilized for focusing. Focusing accuracy is higher with this method than with the image split method. For example, matching accuracy by naked eye at 250 mm was measured to show that separate angles at 250 mm can be recognized with an accuracy of .+-.5 seconds and .+-.6.5 .mu.m using the image sandwiching method, and with .+-.8 seconds and .+-.10 .mu.m using the image split method.
A conventional optical system using the image sandwiching method will be described referring to FIG. 10.
Light from a light source 80 proceeding along its optical axis is half-reflected by a half reflection mirror 82 and then converged by a lens 86 and sent through a double slit mask 90 so that double sight lines are formed. The sight lines then pass through a projection lens 94 and proceeding to a mask 91, which masks the lower half of the passing light. The upper half the light passes through the mask 91 and is reflected by a full reflection mirror 84. Light from the light source 80 passing through the half reflection mirror 82 is meanwhile reflected by full reflection mirror 84 and converged by a lens 87. This light then passes through a single slit mask 92 to form a single sight line. The line then passes through a projection lens 96, proceeding to a mask 93. As the upper half of the passing light is shielded by the mask 93, only the lower half passes through the mask 93, and is half-reflected by another half reflection mirror 82. With this arrangement, images are formed in which the single sight line is sandwiched by the double sight lines.
In this image sandwiching method, however, the whole system has a complicated structure including two optical systems, and adjustment thereof is thus troublesome.
Various other automatic focusing devices have been proposed. They typically employ a method in which light from light emitting elements is reflected at the surface of a workpiece; the reflected light is received by light receiving elements; and whether a workpiece is positioned either closer or further with respect to a focal point is determined based on the received light.
In such a method, however, the condition of the surface of a workpiece is likely to influence focusing accuracy, as light from light emitting elements is reflected at the surface. Even more troublesome, a focused state cannot be observed by the naked eye.